This invention relates to golf clubs, and, more particularly, to iron golf clubs which are designed to have gear effect.
Gear effect in wooden clubs is well known. When a wooden club, e.g., a driver, strikes a golf ball at a point which is offset from the center of the face, i.e., toward the toe or heel of the club, a spin is imparted to the ball. A toe hit on a right-handed club will provide a counterclockwise or hook spin, and a heel hit on a right-handed club will provide a clockwise or slice spin.
The gear effect spin is created by a wooden club because the center of gravity of the club is spaced a substantial distance behind the striking face of the club. When the club strikes a ball on an off-center hit, the clubhead rotates about its center of gravity. The rotation of the clubhead in one direction, e.g., clockwise for a toe hit, causes the ball to rotate in the opposite direction, i.e., counterclockwise for a toe hit. An opposite rotation is imparted to the ball because the clubhead and ball rotate together much like two enmeshed gears. On a heel hit, the club rotates counterclockwise, and a clockwise or slice spin is imparted to the ball.
The clockwise rotation of a wooden clubhead on a toe hit opens the face of the club and causes the ball to fly initially to the right of the intended line of flight. However, the counterclockwise or hook spin imparted to the ball by the gear effect will cause the ball to curve back toward the intended line of flight. In most wooden clubs the gear effet spin more than compensates for the effect of the open face, and the ball would hook to the left of the intended line of flignt. For this reason the striking face of a wooden club is provided with "bulge," i.e., a curved or convex face. The bulge tends to make a toe hit fly to the right and a heel hit fly to the left. The bulge spin and the gear effect spin are advantageously adjusted so that a ball hit on either the toe or the heel lands approximately along the intended line of flight.
Most iron clubs do not impart appreciable gear effect spin to off-center hits. A ball hit on the toe of an iron will cause the clubhead to open or to rotate clockwise. However, since the center of gravity of most irons, particularly forged irons, is located very close to the striking face of the iron, very little gear effect spin will be imparted to the ball. The open face will tend to create s slice spin on the ball. Conversely, a heel hit on a conventional iron will close the face, and tend to create a hook spin.
The gear effect in wood clubs has been well known and understood for many years, and there have been suggestions to incorporate gear effect in irons. However, to the best of my knowledge no iron has been intentionally designed to impart gear effect corrective spin to a ball on off-center hits so that an off-center hit will curve back to the intended line of flight, thereby improving the accuracy of off-center hits.
I have found that optimum gear effect in irons is a function not only of the location of the center of gravity behind the striking face but also of the moment of inertia and radius of gyration of the clubhead about the center of gravity. Simply positioning the center of gravity of an iron a certain distance behind the face of the club will not necessarily produce the desired results. For example, the center of gravity of a club can be relatively far behind the face, but if the moment of inertia or radius of gyration is too low, then the club will rotate too much. The excessive rotation can cause either too much gear effect spin or too much of a tendency for the ball to fly in the direction in which the club rotates.
In accordance with the invention, the dimensions of iron clubs are selected to optimize the position of the center of gravity, the moment of inertia, and the radius of gyration so that a ball hit on the toe or the heel will tend to curve back to approximately the intended line of flight. The irons are advantageously investment cast to provide a cavity in back of the center of the striking fact so that most of the weight of the clubhead is positioned at the sole, toe, and heel portions of the club. The weight at the toe and heel increases the moment of inertia and the radius of gyration of the clubhead about the center of gravity, and the wide sole, toe, and heel cause the center of gravity to be located well behind the face. The spacing of the center of gravity behind the face of the clubs in a set progressively increases as the loft angle of the clubs decreases, the moment of inertia of the clubs in a set progressively decreases as the loft angle of the clubs decreases, but the radius of gyration of the clubs in the set is relatively constant.